Currently, information is displayed using assembled sheets of paper carrying permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Printed information cannot be changed. Electrically updated displays are often heavy and expensive. Other sheet materials can carry magnetically written areas, for example, to carry ticketing or financial information. Such magnetically written data, however, is not visible.
Media systems exist that maintain electronically changeable data without power. Such system can be electrophoretic (Eink), Gyricon, or polymer dispersed cholesteric materials. An example of such electronically updateable displays can be found in U.S. Pat. No. 3,600,060 issued Aug. 17, 1971 to Churchill et al., which patent shows a device having a coated, then dried emulsion of cholesteric liquid crystals in aqueous gelatin to form a field-responsive, bistable display. U.S. Pat. No. 3,816,786 also to Churchill et al. discloses a layer of encapsulated cholesteric liquid crystal responsive to an electric field. The electrodes in the patent can be transparent or non-transparent and formed of various metals or graphite. It is disclosed that one electrode must be light absorbing, and it is suggested that the light absorbing electrode be prepared from paints contains conductive material such as carbon.
Fabrication of flexible, electronically written display sheets is disclosed in U.S. Pat. No. 4,435,047 issued Mar. 6, 1984 to Fergason. A substrate supports a first conductive electrode, one or more layers of encapsulated liquid crystals, and a second electrode of electrically conductive ink. The conductive inks form a background for absorbing light, so that the information-bearing display areas appear dark in contrast to background non-display areas. Electrical potential applied to opposing conductive areas operates on the liquid crystal material to expose display areas. Because the liquid crystal material is nematic liquid crystal, the display ceases to present an image when de-energized, that is, in the absence of a field. The patent discloses the use of dyes in either the polymer encapsulant or liquid crystal to absorb incident light. The patent further discloses the use of a chiral dopant. The dopant improves the response time of the nematic liquid crystal, but does not cause the nematic host to operate in a bistable light-reflective state.
U.S. Pat. No. 5,251,048 to Doane et al. discloses a light-modulating cell having a polymer-dispersed chiral-nematic liquid crystal. The chiral-nematic liquid crystal has the property of being electrically driven between a planar state, reflecting a specific visible wavelength of light, and a focal-conic state, transmitting forward scattering light. Chiral-nematic liquid crystals, also known as cholesteric liquid crystals, potentially in some circumstances have the capacity of maintaining one of multiple given states in the absence of an electric field. Black paint can be applied to the outer surface of a rear substrate to provide a light-absorbing layer forming a non-changing background outside of a changeable display area defined by the intersection of segment lines and scanning lines.
U.S. Pat. No. 5,636,044 to Yuan et al. discloses a seven-segment display, using cholesteric liquid-crystal material, which display has two substrates. The substrates are rigid glass with patterned transparent electrodes on each of two facing surfaces. A continuum of cholesteric liquid crystal fills the gap between the two electrode sets. The first substrate is divided into segmented and non-segmented areas which are defined by gaps in transparent, electrically conductive Indium-Tin-Oxide (ITO) disposed on the substrate. The second substrate is divided into common electrodes in an ITO coating corresponding to segmented and non-segmented areas on the first substrate. The device can change the state of the segmented areas as well as non-segmented areas, permitting the display of a positive or negative image. Both electrodes are transparent electrodes, requiring an additional light-absorbing layer on the back of one substrate. Inter-segment material, or gaps in the electrode materials, requires electrode contacts to each segment area to write; requiring separate electrical connection to each segment area. It would be useful to have a structure that could provide simple electrode connect to each segment area, in a matrix fashion, without requiring point connection to each segment area.
U.S. Pat. No. 6,236,442 to Stephenson et al. discloses a display sheet with a metallic conductive layer over a cholesteric layer. A process is disclosed for vacuum depositing a continuous metallic layer and laser patterning the metallic layer to form segment electrodes. Moralized areas remain between electrically driven areas. However, those areas between etched segments remain in an as-coated state. A circuit board with contacts is pressed against each segment electrode. The circuit board provides electrical drive to segment electrodes.
U.S. Pat. No. 6,394,870 to Petruchik et al discloses an opaque conductive material with non-conductive areas to delineate images stored in polymer-dispersed cholesteric liquid crystal. The patent discloses printing the opaque conductive material, and providing contact to each image-bearing area. No conductive material is provided between segments. In one embodiment, a dielectric area with openings to each opaque conductive area is printed over the opaque conductive material. Traces are then printed over the dielectric layer to eliminate a circuit board with contacts. Areas between etched segments remain in an as-coated state.
There is a need for a display using polymer-dispersed cholesteric liquid crystals having background material with optical properties matching optical properties of the written segments.